Apparatus and method for carbon fiber surface treatment

ABSTRACT

An apparatus and method for enhancing the surface energy and/or surface chemistry of carbon fibers involves exposing the fibers to direct or indirect contact with atmospheric pressure plasma generated using a background gas containing at least some oxygen or other reactive species. The fiber may be exposed directly to the plasma, provided that the plasma is nonfilamentary, or the fiber may be exposed indirectly through contact with gases exhausting from a plasma discharge maintained in a separate volume. In either case, the process is carried out at or near atmospheric pressure, thereby eliminating the need for vacuum equipment. The process may be further modified by moistening the fibers with selected oxygen-containing liquids before exposure to the plasma.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/159,006 filed Jun. 22, 2005 which claims the benefit of ProvisionalPatent Application No. 60/582,869 filed on Jun. 24, 2004 by the presentinventors, the entire disclosure of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.DE-AC05-00OR22725 awarded by the U. S. Department of Energy toUT-Battelle, LLC, and the Government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to apparatus and methods to treat carbon andgraphite materials, and more particularly to systems and methods forplasma assisted treatment of carbon fibers to improve their surfacecharacteristics for use in polymer matrix composites.

2. Description of Related Art

Carbon fiber composites require that the carbon fibers be bonded to aresin to form a structurally sound composite. Virgin (untreated) carbonfibers are well known to have a low chemical affinity to resins andother polymeric based materials. Usually the carbon fiber must betreated to increase the amount of chemical affinity to the matrix resin.The adhesive characteristics of the resin to the carbon fiber aredominated by the′ surface energy of the carbon fiber. Through surfacetreatment, the addition of oxygen atoms to the surface of the carbonfiber has been demonstrated to increase the surface energy of the carbonfiber. Standard industrial processes for increasing the surface energyof carbon fiber are the use of gaseous oxidative species (ozone) or aliquid electrostatic technique. These common techniques result in boundoxygen concentrations of ˜6%. There are other techniques, i.e. chemicalcoupling agents, liquid oxidizing agents, low pressure gas plasma,chemical solutions, vapor phase deposition, chemical etching, andelectropolymerization; however these processes are not commonly used inindustry.

The use of oxygen-containing plasmas to modify the surface of carbonfibers has been previously examined. The advantages are: increasing thelevel of oxygen onto the surface; increasing the surface energy;providing an oxidative treatment of the top surface creating a differenttype of oxidative chemical groups; and improving adhesion with resin byimproving the chemical affinity of the resin and fiber surface. Thedisadvantages are: undesirable mechanical surface effects; surfaceablation, etching, erosion, or pitting leading to the creation oflocalized surface irregularities on the carbon fiber; removal of theoutmost structures and morphology; general structural damage to thefiber; and reduced mechanical strength.

U.S. Pat. No. 6,514,449 teaches the use of microwave energy and plasmato modify the surface topography of carbon fiber. General discussions offiber surface modification by plasma processing may be found in severalreferences [Mittal, K. L. and Pizzi, A., “Adhesion Promotion Techniques.Technological Applications,” Marcel Dekker, pp. 67-76 and pp. 139-73.(1999); J. B. Donnet, T. K. Wang, S. Rebouillat and J. C. M. Peng,“Carbon Fibers,” Third Edition, Marcel Dekker, Inc., pp. 180-9 (1998)].These teachings address the modification of surface morphology orsurface chemistry as a means of modifying interactions between fiber andmatrix in a composite.

Low-pressure (3-5 Torr) plasma processing has been described for severaldifferent operations relating to carbon fiber production (see Paulauskaset al., U.S. Pat. No. 6,372,192), In this patent it was suggested thatsmall amounts of oxygen could be admitted into a low-pressure microwaveplasma to achieve some surface treatment of the fibers.

It is widely recognized that low-pressure plasma processes are lessdesirable to industrial operations because of the high cost, space, andmaintenance requirements associated with vacuum systems [see C.-M. Chan,Polymer Surface Modification and Characterization, Hanser Pub., 1994,pp. 225-63]. A viable plasma-based process that could operate at nearambient pressures while achieving adequate modification of the fiberswas therefore needed and was not available based on prior teachings inthe literature.

Objects and Advantages

Objects of the present invention include the following: providing anapparatus for treating the surface of carbon fibers prior to theirincorporation into composites; providing an apparatus for more rapid andcost-effective treatment of carbon fibers by exposure to reactiveoxidative species; providing a method for carbon fiber treatment that isfaster and requires less physical space; providing a method for carbonfiber treatment that does not require a vacuum or low-pressure plasma;providing a method of carbon fiber treatment in which the fiber iscontacted by a diffuse plasma in order to avoid the deleterious effectsof a concentrated or filamentary plasma on the fiber; and, providing amethod for treating carbon fibers by exposing them to reactive speciesoriginating in a plasma discharge without exposing the fibers to contactwith the plasma itself. These and other objects and advantages of theinvention will become apparent from consideration of the followingspecification, read in conjunction with the drawings.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an apparatus for treatingcarbon fiber comprises: a treatment chamber adapted to maintainsubstantially one atmosphere pressure; a plasma discharge containingselected gases; and, a means for supporting the carbon fiber within thetreatment chamber at a location wherein the fiber will not contact afilamentary plasma discharge but will be exposed to desired reactivespecies generated by the plasma, whereby the surface of the fiber ismodified through the exposure in the chamber at a selected temperature.

According to another aspect of the invention, a method for treatingcarbon fiber comprises exposing the fiber at a selected temperature andsubstantially one atmosphere pressure to a gas containing at least onereactive species, the reactive species generated by a plasma discharge,while avoiding direct contact between the fiber and a filamentary plasmadischarge, whereby the surface energy of the carbon fiber is modified byexposure to the reactive species.

According to another aspect of the invention, an apparatus for treatingcarbon fiber comprises a treatment chamber having a first volume adaptedto contain the carbon fiber during treatment at substantially oneatmosphere pressure and a second volume wherein a plasma is established;a source of selected gases into the second volume, whereby reactivespecies may be created within the plasma; and, at least one conduitbetween the first and second volumes whereby the reactive species fromthe plasma may be transported to the fiber and the fiber surface energyis modified by exposure in the chamber at a selected temperature whileavoiding direct contact between the fiber and the plasma.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. A clearerconception of the invention, and of the components and operation ofsystems provided with the invention, will become more readily apparentby referring to the exemplary, and therefore non-limiting embodimentsillustrated in the drawing figures, wherein like numerals (if they occurin more than one view) designate the same elements. The features in thedrawings are not necessarily drawn to scale.

FIG. 1 is a schematic diagram of one embodiment of the present inventionadapted for atmospheric pressure plasma processing.

FIG. 2 is a schematic diagram of three alternate designs for exposingthe fiber tow to plasma-derived reactive oxidative species.

DETAILED DESCRIPTION OF THE INVENTION

In its most general form, the invention is designed to modify thesurface characteristics of carbon fibers by exposing the fibers eitherdirectly within a bulk volume of atmospheric plasma (direct treatment)or to a flow of chemically active species originating in an atmosphericplasma (indirect treatment). Properties that can be altered include thesurface energy, number and type of chemically active binding sites, thesurface area and surface roughness. Carbon fibers are placed within aplasma device capable of generating a sufficient volume of ionized gas,in a manner that minimizes filamentary current structure within theplasma. The fiber is maintained within the plasma for a period of timeto allow for the alteration of the surface properties, typically tens ofseconds but generally less than tens of minutes. The plasma is generatedin such a manner as to promote the production of chemical speciesnecessary for the desired surface alteration.

Direct Exposure Plasma Treatment

To modify the surface energy, increase the surface area, and roughen thesurface of the fiber: The fiber is pulled or placed into an atmosphericplasma device exposing the fiber to direct contact with the chemicalactive species generated by and comprising the plasma. The atmosphericpressure plasma device is configured to operate in a non-filamentarymode using a background gas preferably comprising air or any otheroxygen containing gas mixtures including pure oxygen. The plasmaoperating conditions are adjusted to yield the desired surfacemodifications within the required residence time. Typical adjustableparameters include the size of the plasma volume, the composition of theprocessing gas, gas flow rates, and the energizing conditions of theelectrical device generating the atmospheric plasma. These simpleadjustments in the operating parameters allow for the generation ofselected chemical species responsible for particular surfacemodifications and for varying degrees of surface topography effects.

Indirect Exposure Plasma Treatment

To modify the surface energy of the fiber, and alter the number and typeof chemically active sites on the surface of the fiber: The fiber ispulled or placed into the exhaust flow from an atmospheric plasma deviceexposing the fiber to contact with the convected chemical active speciesgenerated by and comprising the plasma. The atmospheric pressure plasmadevice is configured to operate using background gas preferablycomprising air, or any other oxygen containing gas mixtures includingpure oxygen, that promotes the transport of short-lived reactiveoxidative species to the fiber via a sufficiently high exhaust velocity.The plasma operating conditions are adjusted to yield the desiredsurface modifications within the required residence time. Typicaladjustable parameters include the size of the plasma volume, thecomposition of the processing gas, gas flow rates, and the energizingconditions of the electrical device generating the plasma. These simpleadjustments in the operating parameters allow for the generation ofselected chemical species responsible for particular surfacemodifications. Deleterious effects on fiber surface topography may beminimized by the indirect exposure process because the fibers arelocated away from the bulk of the plasma and do not undergo directionbombardment.

The oxidative chemistry required to carry out the surface modificationis fundamentally complex, so the addition of plasma processing increasesthe degree of complexity of the overall process kinetics. This addedcomplexity arises from the generation of gaseous concentrations of bothshort-lived and excited gas species as well as energetic photons. Theadditional reactive species and photons dramatically alter the overallreaction kinetics governing the oxidative processes. Experimentaltesting with a variety of gas mixtures confirmed the importance ofincluding oxygen during the process.

It will be understood that although air is a preferred working gas forthe plasma device, other mixtures of processing gases comprising someoxygen mixture or oxygen containing gas may be suitable for particularapplications. Examples include, but are not limited to: dry or moistair; nitrogen; oxygen; nitrogen oxides; carbon dioxide; helium, argon,or other inert gases; hydrogen and hydrogen-containing gases includingammonia; and mixtures thereof. Reactive species that may be created bythe plasma source include the following: O₂, O_(x), O, N_(x)O_(y),H_(x)O_(y), as well as ions, radicals, excited states and metastables ofany of the precursor gases.

As noted above, generally surface treatment is an oxidative process;however, other useful modification processes generally referred to as“grafting” may require a substantially oxygen-free plasma. Plasmagrafting is a technology by which surface properties of a polymer orcarbon fiber can be tailored through the proper selection of gases togenerate very specific chemical groups onto the surface of carbon fiber,making the fiber more compatible with a specific matrix resin. For theseapplications, useful gases may include the following: light hydrocarbons(C-1 through C-8), which may be saturated or unsaturated, open chain orcyclic; NH₃ and other amines; H₂; alcohols; organic acids; ethers;ketones, esters; and aldehydes.

A variety of atmospheric pressure plasma devices have been built duringthe course of the present research effort. These devices are generallybased on the approach taught in U.S. Pat. Nos. 5,387,842 and 5,414,324.These devices have consisted of various parallel electrode designs thathave included parallel plates and rods. In these designs either one orboth of the electrodes are insulated. The processing gas and carbonfiber was fed through a gap or spacing between the electrodes. Theelectrodes were energized with an audio frequency power supply andoperated in such a manner as to promote a diffuse dielectric barriertype discharge.

The atmospheric pressure plasma processing equipment consists of a gassupply system, a plasma reactor, and a high voltage power supply asshown in FIG. 1. The gas supply system consists of a gas manifold 39with connections for up to four different gas bottles 37 (two of whichare shown) with independent meters 38. The gas manifold deliverstemperature controlled, typically heated by heater 60, mixed gas to theplasma reactor. The plasma reactor consists of temperature controlledelectrodes 67, (via a heated oil recirculated through their interiorsthrough inlets 63 a, 63 b and outlets 64 a, 64 b), dielectrics 34, 35,and an enclosure 66. Dielectric materials included borosilicate glass,quartz, and alumina. The enclosure 66 for the plasma reactor isnecessary both to manipulate the processing gas into and to exhaust gasout of the gap between the electrodes where the plasma is formed and thecarbon fibers were processed. The high voltage system 31 consists of anaudio amplifier or power inverter coupled to a high voltage transformer.The high voltage from the transformer is connected via high voltagewires 32 to the electrodes within the plasma reactor. Electricaldiagnostics consisting of a high voltage probe and current ringconnected to an oscilloscope 33 monitor the electrical parameters ofpower supply.

The plasma device used in the following examples consisted of a twoinsulated parallel plates that were enclosed by a first set of wallsused to confine the background gas and set the spacing between theelectrodes and a second enclosure used to confine the exhaust gasesprior to being drawn into the building exhaust system. The originalsystem had oil-heated electrodes, were insulated with Pyrex glass platesand had a plasma gap size smaller than 3 millimeters. The fiber and thelower electrode where electrically connected together either to ground,with the upper insulated electrode connected to the high voltage, or tothe high voltage with the upper insulated electrode connected to ground.

Nominal results for direct and indirect atmospheric plasma treatment ofcarbon fiber are presented in the following examples. In order toeliminate the effects of fiber type, the base material for all tests wasthe same untreated, unsized, conventionally manufactured PAN basedcarbon fiber: large tow (50 k filaments), FORTAFIL F3(C)™ ContinuousCarbon Fiber [Fortafil Fibers, Inc., 121 Cardiff Valley Road, Rockwood.Tenn. 37854]. It will be appreciated that the inventive process is notlimited to any particular type of carbon fiber, but may be applied tocarbon fibers derived from polyacrylonitrile (PAN), pitch, rayon,lignin, cellulose, or other thermoplastic-based fiber materials.

Chemical surface analysis on the carbon fibers was performed using X-rayPhotoelectron Spectroscopy (XPS). Each average result represents aminimum of five to seven measurements taken from different positionsalong a sample of fiber. Each measurement taken at each position was anaverage of 16-20 individual XPS scans. Tables reflect XPS surfaceanalysis data for surface elementary oxygen concentration only.

EXAMPLES Example 1 Direct Exposure

Carbon fibers (unsized) were placed on a dielectric support in aparallel-plate atmospheric plasma reactor substantially as shown inFIG. 1. A voltage of 7 kV at 8 KHz was applied to the fiber with thewalls grounded, and the fibers were directly exposed to the plasma. Thesurface of the dielectric was about 90° C. during treatment. Table 1shows exposure time and gas composition for each test. The surfaceoxygen concentration (given as %±STD) was measured freshly afterexposure as well as after 5 weeks of storage under vacuum. Forcomparison, a typical value for unprocessed carbon fiber is about4.4±1.0 and for conventionally processed (ozone treated) fiber it isabout 6.2±1.1. It can be seen that the plasma treated fibers havesubstantially greater surface oxygen content, even after prolongedstorage under vacuum.

TABLE 1 General comparison of atmospheric plasma processed samples andconventionally processed samples. Sample Gas Time, s Surface O (fresh)Surface O (stored) A Dry air 70 23.59 ± 6.23  7.7 ± 0.9 B Dry air 18029.80 ± 5.47 12.7 ± 2.7 C 100% O₂ 70 29.09 ± 9.56  5.3 ± 0.9 D 100% O₂190  20.85 ± 11.38 10.8 ± 1.8 E^(a) 100% O₂ 150 28.06 ± 3.69 18.3 ± 2.3^(a)Fibers were moistened with distilled water before treatment

Example 2 Direct Exposure

A test was conducted to determine the effectiveness of very shortprocessing times. In this test, a cylindrical plate plasma device wasused. This device was cylindrical rather than planar, but its crosssection (taken along the axis of the cylinder) was the same as thatshown in FIG. 2A. A voltage of 7.5 kV at 8 kHz was applied to the fiberand the chamber walls were not heated. The result is shown in Table 2.

TABLE 2 Short processing time in a cylindrical reactor Sample Gas Time,s Surface O (fresh) 2 Air^(a) 75 8 ± 0.8 ^(a)Synthetic dry air.

Example 3 Direct Exposure

The effect of longer processing time was investigated using a setupidentical to that described in Example 1. The results are shown in Table3.

TABLE 3 Long processing time in a parallel plate reactor Sample GasTime, s Surface O (fresh) Surface O (stored) 3 Air 180 12.7 ± 2.7 8.3 ±1.6 4 100% O₂ 190 10.8 ± 1.8

Example 4 Direct Exposure

Considering the high electrical conductivity of the carbon fiber, thefiber itself can be used as one of the electrodes either in the plasmadevice or biased downstream of a plasma device. Biasing the fibers to aparticular potential may affect the type of ions that are preferentiallydriven to the surface of the fiber. In this test, the cylindricalreactor described in Example 2 was used. Table 4 shows data for samplesprocessed within the plasma with the fibers either connected toelectrical ground or energized by the high voltage used to generate theplasma discharge.

TABLE 4 Effect of electrical bias on fiber processing time in a parallelplate reactor Sample Gas Time, s Sample bias Surface O (fresh) 5 100% O₂75 ungrounded 6.8 ± 1.2 6 100% O₂ 75 grounded 3.7 ± 1.2

It will be appreciated by those skilled in the art that the sample maybe biased to any selected potential from ground potential to energizedas shown in the example, as well as given a DC bias at any selectedvoltage (for example, as a means of attracting or repelling particularchemical species within the plasma). It will further be appreciated thatany such bias applied to the fiber may be held constant throughout theprocess or may be changed as the process continues in order to adjust tochanging fiber properties or to provide a degree of dynamic control ofthe overall process kinetics.

Example 5 Direct Exposure

The use of liquid agents to wet the surface of the fiber prior to directexposure to the plasma may allow a greater quantity of oxygenatedspecies to be preferentially located at the surface of the fiber.Dissociation of the liquid during plasma processing releases the oxygenspecies. Clearly, a small quantity of water or other oxygen containingliquid stoichiometrically provides a large quantity of oxygen in thevicinity of or even directly onto the surface of the fiber. Water,commonly thought to be of negative impact to the surface energy duringthe carbon fiber production, is an example of a liquid that can be usedto increase the surface energy. Several runs were made using fiber thathad been slightly moistened with distilled water immediately beforeexposure to the plasma, in a parallel plate reactor setup as describedin Example 1, and in a cylindrical reactor as described in Example 2,and the results are shown in Table 5. It will be appreciated that thisprinciple may be extended to other oxygen containing liquids for similarbenefits. Examples of oxygen containing fluids include water and otherlow molecular weight organic compounds containing single or multiplecarbonyl, carboxyl, alcohol, ester, or ether groups. The preference forlow molecular weight liquids is based on two considerations. First, ifthe viscosity of the liquid is too high, it will be difficult to wet thefibers with a thin, uniform layer of the liquid. Second, as themolecular weight increases, the amount of oxygen decreases relative toother constituents (typically carbon) and so the usefulness of the fluidto deliver oxygen is proportionately reduced. It will be clear that theactual type and molecular weight of such fluids will vary to some degreewith particular applications, but Applicants prefer to use a fluid whoseviscosity is less than about 500 centipoise (cp). It will be appreciatedthat the viscosity of a selected liquid will vary as the processtemperature varies. Thus, the choice of liquid will depend to somedegree on the choice of process temperature. Other compositemanufacturing processes requiring a particular matrix resin may alsorequire plasma processing gases containing hydrogen/nitrogen i.e.methane or ammonia. The inventive atmospheric plasma surface treatmentmay be further modified by the addition of these species to the plasmain order to generate the selected chemical groups on the fiber surface.

TABLE 5 Plasma processing using pre-moistened fibers Sample Gas Time, sSurface O (fresh) Surface O (stored) 7^(a) 100% O₂ 150 18.3 ± 2.3 11.8 ±2.3 8^(b) 80% O₂ 20% 150 10 ± 2 N₂ ^(a)7 kV at 8 kHz applied to fiber,walls grounded, parallel plate reactor ^(b)7.5 kV at 8 kHz applied tofiber, cylindrical plate reactor

Example 6 Indirect Exposure

In some applications it may be desirable to avoid exposing the fiberdirectly to the plasma (to minimize sputtering effects, for example). Toexamine the effectiveness of indirect exposure, a cylindrical version ofthe parallel plate configuration used in direct exposure was used inconjunction with a rotating sample holder attached downstream of theexit. The plasma was generated in air using 12 kV at 6 kHz. The flow ofinlet air was 2 cfm and the humidity of the air was held constant at 2.5mm Hg. The fibers were exposed to the gas stream exiting the plasmavolume but were not directly exposed to the plasma. As before, multipleXPS scans were made on the fibers and the results are presented in Table6.

TABLE 6 Indirect processing using gas stream from a plasma generatorSample Gas Time, s Surface O (fresh)  9 Air 90 9.6 ± 1.4 10 Air 180 9.1± 1.5 11 Air 180 11.9 ± 1.1 

The use of vacuum plasma processing of carbon fiber (either via directexposure or remote exposure) for surface energy enhancement is wellknown in the art. It will be appreciated that Applicants' use ofatmospheric plasma simplifies the production process by removing theusual batch-processing requirement created by vacuum plasma processingequipment. The inventive process achieves surface energy values higherthan the standard industrial processes without the use of vacuum-basedprocessing equipment. A preferable approach to generating atmosphericplasma is the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP) oratmospheric diffuse mode dielectric barrier type plasma because thisallows for a more efficient use of the plasma volume as taught in U.S.Pat. Nos. 5,387,842 and 5,414,324. Other types of plasma generators maybe used in other embodiments of Applicants' process. In addition to thevolumetric efficiency of these spatially uniform plasmas, thenon-filamentary character of these plasma discharges enhances control ofthe plasma etching to insure a uniform degree of treatment of eachindividual carbon fiber filament within the tow. Furthermore, theatmospheric plasma processing system eliminates any effluent that iscommonly created from an electrostatic process. While ozone may bepresent within the gas chemistry of the inventive process and devices,the results achieved are significantly higher than that achieved withozone alone.

In order to provide a better understanding of the theoretical basis forApplicants' approach, the following discussion of filamentary versusnon-filamentary atmospheric pressure plasma discharges is provided.[Reference is made to Rahel, J., and Sherman, D. M., “The transitionfrom a filamentary dielectric barrier discharge to a diffuse barrierdischarge in air at atmospheric pressure”, J. Phys. D: Appl. Phys. 38(2005), the entirety of which is incorporated herein by reference.] Asknown by those skilled in the art of plasma physics, atmosphericpressure plasma discharges can be classified into two categories basedon the fundamental nature of the plasma discharge. Filamentarydischarges are composed of microfilaments of current, pinpoint-like incharacter, that have localized energies sufficiently high to causedamage to delicate substrates in contact with the filamentary plasmadischarge. Uniform treatment of a sample placed in such dischargerequires some means of spatial averaging, and even then there is atendency to overly damage the sample. Examples of filamentary plasmadischarges would include corona, dielectric barrier discharges, and slotor micro-hollow cathode discharges. Atmospheric pressure non-filamentarydischarges are plasma devices in which the pinpoint-like character ofthe current within the plasma is absent. This can be accomplished by avariety of mechanisms, such as operating the plasma comprising inertgases, operation within an after-glow of a plasma device, or tailoringthe plasma discharge characteristics such that the filamentary structureis suppressed or diffused as taught in U.S. Pat. Nos. 5,387,842 and5,414,324. Examples of non-filamentary or diffuse plasma dischargeswould include low pressure glow discharges, diffuse barrier discharges,the after-glow of atmospheric pressure jet discharges and similardischarges.

Distinguishing between a filamentary plasma discharge and anon-filamentary discharge in most cases can be accomplished by visualinspection if the plasma device is configured such that the one or bothof the electrodes is transparent. Using a transparent electrode allowsfor the light emission from the plasma to be seen both the human eye andby a photomultiplier tube. Both the current waveforms and the photonemission for both a filamentary dielectric barrier discharge (FDBD) anda diffuse (non-filamentary) barrier discharge (DBD) contain featuresthat may be used to readily distinguish the two types of discharge, asdiscussed in detail in the cited reference [Rahel and Sherman (2005)].

While the results of vacuum based plasma processing could be similar toatmospheric pressure based plasma system, the advantages of Applicants'atmospheric pressure process include: lower capital and operating costs;easier implementation in an industrial environment; reduced need forvacuum seals; and easier adaptation to continuous processing.

There are many possible ways of delivering the reactive species from theplasma to the fiber material. As previously noted, these designs can becategorized by either placing the fiber within or outside of the plasmadischarge that is either volumetrically generated between two electrodesor a surface discharge such that the plasma is a thin volume of plasmaabove a surface of dielectric. Those skilled in the art will appreciatethat the inventive process may alternatively be accomplished by placingthe fiber in the vicinity immediately outside of the plasma volume aswell as within the smaller volume of a surface discharge. The advantageof placement in close proximity to, but not within, a plasma dischargeis that the plasma could be operated at even higher energy levels thatwould otherwise create unacceptable amounts of detrimental etching ofthe fiber. These higher energy levels would allow for greaterconcentrations of reactive species and promote more rapid treatment ofthe fiber. The advantage of a surface discharge for fiber processingeither within or immediately downstream of the surface plasma is thatthe plasma surface has higher electric field strengths, is easier toaccess, and is considerably simpler to maintain. FIG. 2 showsschematically several design variants. In each case, the figure shows across-sectional view of a substantially planar geometry.

FIG. 2A illustrates a fiber tow 3 passing between parallel electrodes 51with dielectric layers 52. In this case, the fiber passes through theplasma 54, wherein it is directly exposed to reactive species. Theplasma 54 is non-filamentary in nature.

FIG. 2B illustrates a similar electrode and dielectric configuration asshown in FIG. 2A; however, in this case the fiber consists of a flatcoil of tow 3′ disposed outside of the plasma 54′. Reactive speciesarising in the plasma 54′ are convected out of the plasma and come intocontact with the fiber 3′. Because the fiber is not directly immersed inthe plasma 54′, this plasma may be filamentary or non-filamentary innature.

FIG. 2C illustrates another configuration, in which a series of parallelelectrodes 51′ are embedded in a planar dielectric 52′ and create asurface plasma discharge 54″. If the surface plasma is diffuse(non-filamentary), the fiber 3 may be immersed within the plasma asshown. Alternatively, if the plasma is filamentary the fiber may bedisplaced upward so that it lies outside of the plasma discharge butstill dose enough to become exposed to the reactive species originatingwithin the plasma 54″.

Alternate Embodiments for Direct Exposure

The high electrical conductivity and small diameter of the individualfibers within the fiber bundle can create many challenges to a plasmaprocessing system, particularly in electrically shorting the electrodesystem. While an insulated parallel plate arrangement is preferred forscaling up to a large number of fiber tows, it may be desirable tocontain the fibers in a dielectric cavity as they are drawn through theplasma device. Such confinement will mitigate electrical shorting of theplasma generating electrodes.

Additionally, an array of tubular geometric electrodes with insulatedinteriors is also feasible. In this case the fiber bundles are dividedand passed through the insulated interiors, with the conductive fiberelectrically connected to comprise one of the required high voltageelectrodes while the insulated tubular electrodes comprise the otherone.

With the assumption of a well-insulated system, other geometricconfigurations of the electrodes are also feasible. These otherconfigurations would include, but are not limited to, rods to flatplate, rods to rods, concentric plates, and various surface dischargeconfigurations. Furthermore, assuming the proper electric field isgenerated that leads to the formation of the plasma, only one electrodehas to be insulated assuming that the fiber has the same potential asthe non-insulated electrode.

Alternate Embodiments for Indirect Exposure

There are a large number of electrode configurations that could beemployed to generate the plasma exhaust required to treat the fiber. Theprimary requirements are sufficient exhaust velocity to transport theshort-lived active species, sufficient gas residence time within theplasma, and the proper electrical operating conditions sufficient forcreating the required short-lived active species. It will be appreciatedthat the carbon fiber must be located at a distance from the plasmasource wherein the desired concentration of reactive species exists inthe exhaust stream; the optimal location may be determined throughroutine experimentation. Useful plasma device configurations include,but are not limited to, flat plate to flat plate, rods to flat plate,rods to rods, concentric plates, and various surface dischargeconfigurations as are familiar to those skilled in the art of plasmageneration. The requirement for non-filamentary plasma discharges mayalso be relaxed for indirect plasma exposure because the fiber isphysically located outside the boundary of the plasma discharge andtherefore safely away from the destructive filaments.

It will be appreciated that the carbon fibers may be supported duringthe process by a number of different methods. As used herein, the term“means of support” can include supporting structures that may resideeither inside or outside of the treatment chamber. For a batch-typeprocess, the preferred support will be a generally flat surface uponwhich the fiber material may rest, the surface preferably being adielectric material. For a continuous process, the preferred supportcomprises a feed reel and a take-up reel, both generally located outsideof the treatment chamber. In this case the fiber is suspended betweenthese reels and preferably held in a controlled state of tensionpreventing it from touching or dragging on the internal surfaces of thetreatment chamber. As is well known in the art, the reels may further bedisposed to spread a fiber tow from its naturally cylindrical shape intoa generally flat or ribbon-like configuration for better management ofheat and gas flow around the fibers. It will be understood that theentire process may be conducted with the fiber following a substantiallyhorizontal path as shown in the examples or with the path orientedvertically if desired (for example, to reduce the amount of floor spaceoccupied by the equipment). The system may further be configured toallow for the fiber to make more than one pass through the treatmentchamber if desired. In another embodiment, a continuous conveyor systemmay be provided, for example, to move a chopped fiber product throughthe treatment chamber.

It will be appreciated that the optimal temperature for carrying out theinventive process will vary somewhat depending on the type of fiber, theintended polymer matrix, and other engineering considerations. Ingeneral, the process may be carried out from about 20 to 300° C. andmore preferably from about 20 to 150° C.

Applicants do not regard the particular embodiment of the plasmaconfiguration as critical except that processing of the fiber itemwithin the plasma discharge must be accomplished within anon-filamentary plasma discharge. An alternate means of achieving thiscondition is to process the workpiece in either the after-glow orexhaust of a plasma device such that convection of the requiredoxidative chemistry is accomplished in a short enough time to allow theitem to undergo the surface modification. There are many plasma devicesthat one of ordinary skill in the art of plasma science may utilize toaccomplish this process. It will be further appreciated that if theplasma is generated in a first location and the reactive species aretransported from there to the treatment chamber, various incidentalprocesses may be performed on the reactive species, such as heating orcooling the gas stream to a desired temperature prior to its contactingthe fiber. This incidental processing might in some cases change thechemical composition of the reactive species or change the relativeproportions of the various gaseous species. As one example of thisembodiment of the invention, the plasma may be configured to create alarge concentration of ozone, which may then be heated to decompose theozone into atomic oxygen before introducing it into the treatmentchamber. Thus, a conduit provided to carry the plasma-derived speciesinto the processing chamber may contain such well known structures asheaters, heat exchangers, radiative cooling fins, and the like as arefamiliar in the art.

Those skilled in the art of plasma devices and processes will appreciatethat control of the plasma device may involve any or all of thefollowing parameters: voltage; frequency; current; power; and waveformdensity (pulsing or duty-cycle). The selection of a complete process fora particular carbon fiber product and a particular application willinvolve the application of routine engineering analysis to select andoptimize the following system parameters: plasma device and geometry;gas mixture and flow-rates; gas pressure; temperature; processingresidence time; and composition of item undergoing processing.

It will be further understood that the description of exemplaryprocesses using atmospheric pressure plasma processing should not beinterpreted to limit the inventive process to precisely one atmosphere;on the contrary, the claimed process may be performed at any selectedpressure near ambient, which might be somewhat less than or more thanone atmosphere. Furthermore, the temperature of the fiber duringprocessing may be maintained at substantially ambient temperature, thefiber temperature may fluctuate somewhat with exposure to the plasma, orthe fiber may be actively heated or cooled, depending on the needs of aparticular application.

What is claimed is:
 1. A method for treating carbon fiber comprising:biasing a tow of carbon fiber to provide a first electrode; wetting anexposed surface of at least one carbon fiber of said tow of carbon fiberwith an oxygen-containing species in its liquid state prior tosubjecting said at least one carbon fiber to a non-filamentary plasmadischarge; and forming said non-filamentary plasma discharge between thefirst electrode and a second electrode; and exposing theoxygen-containing liquid that is present on the exposed surface of saidat least carbon fiber to said non-filamentary plasma discharge, whereinthe oxygen-containing liquid dissociates to provide at least oneoxidative species that modifies the surface energy of said at least onecarbon fiber to have a greater chemical affinity to polymeric basedmatrixes.
 2. The method of claim 1, wherein the at least one oxidativespecies oxidizes the exposed surface of the at least one carbon fiber.3. The method of claim 1, wherein the exposure of the at least onecarbon fiber to the at least one oxidative species reduces the surfaceenergy of the at least one carbon fiber.
 4. The method of claim 1,wherein the oxygen-containing liquid comprises water or an organiccompound comprising carbonyl, carboxyl, alcohol, ester, or ether groups.5. The method of claim 1, wherein said non-filamentary plasma dischargeis conducted at substantially one atmosphere pressure.
 6. The method ofclaim 1, wherein the oxygen-containing liquid has a viscosity less thanabout 500 cp.
 7. The method of claim 1, wherein the biasing of the towof the carbon fiber comprises applying a bias to the tow of carbon fiberoutside a region where the non-filamentary plasma discharge is formed.8. The method of claim 1, wherein the forming of the non-filamentaryplasma discharge between the first electrode and a second electrode isat 1 atmospheric pressure (atm).
 9. The method of claim 1, whereinexposure of the oxygen-containing liquid present on the exposed surfaceof the at least one carbon fiber to the non-filamentary plasma dischargeroughens the exposed surface of the at least one carbon fiber.
 10. Themethod of claim 1, wherein said non-filamentary plasma discharge isprovided by a plasma processing device comprising a gas supply, a plasmareactor and a power supply, wherein the gas supply provides a workinggas comprising at least one species selected from the group consistingof: air, O₂, N₂, H₂O, CO, CO₂, NH₃, CH₄, nitrogen oxides, He, Ar, C-1 toC-8 hydrocarbons, organic acids, ketones and aldehydes.
 11. The methodof claim 1, wherein said second electrode is an oil-heated electrodeinsulated with glass plates.
 12. The method of claim 1, wherein saidnon-filamentary plasma discharge is selected from the group consistingof low pressure glow discharge, diffuse barrier discharge, after-glow ofatmospheric pressure jet discharge, and combinations thereof.
 13. Themethod of claim 1, wherein the second electrode is a series of parallelelectrodes embedded in a planar dielectric, wherein the planardielectric is a dielectric material selected form the group consistingof borosilicate glass, quartz, alumina, and combinations thereof.
 14. Amethod for treating carbon fiber comprising: applying a plasma dischargeto a gas in a first chamber to produce at least one reactive species;and exposing at least one carbon fiber to the at least one reactivespecies at substantially one atmosphere pressure in a second chamber, inwhich the second chamber is separated from the first chamber and isconnected to the first chamber by a conduit, wherein exposure of the atleast one carbon fiber to the reactive species modifies the surfaceenergy of the at least one carbon fiber to have a greater chemicalaffinity to polymeric based matrixes, wherein the at least one carbonfiber is not exposed to the plasma discharge that is produced in thefirst chamber.
 15. The method of claim 14, wherein the temperature ofthe at least one reactive species is maintained between about 20° C. toabout 300° C. in the conduit prior to the introduction of the reactivespecies into the second chamber.
 16. The method of claim 14, wherein theat least one reactive species is ozone and the ozone is heated in theconduit to facilitate decomposition, wherein the at least one reactivespecies introduced into said second chamber includes atomic oxygen. 17.The method of claim 14, wherein the plasma discharge is a filamentaryplasma discharge or a non-filamentary plasma discharge.
 18. The methodof claim 14, wherein the plasma discharge is established using a workinggas comprising at least one species selected from the group consistingof: air, O₂, N₂, H₂O, CO, CO₂, NH₃, CH₄, nitrogen oxides, He, Ar, C-1 toC-8 hydrocarbons, organic acids, ketones and aldehydes.
 19. The methodof claim 14, further comprising applying an oxygen-containing liquiddirectly to at least one carbon fiber before exposing at least onecarbon fiber to the at least one reactive species.
 20. The method ofclaim 14, wherein the plasma discharge is provided by a plasma deviceconsisting of flat plate to flat plate, rods to flat plate, rods torods, concentric plates, and combinations thereof.